Geothermal energy has been use for centuries to satisfy general heating requirements. The modern geothermal plant is powered by production wells drilled to a source rock to produce steam at the surface. Depending on the location and depth, source formation temperatures vary.

In Italy, the operator must penetrate very hard and abrasive sediment and metamorphic formations to access steam in the granite basement formation. Historically, this was accomplished with a tungsten carbide insert (TCI) roller cone bit (RC). Standard geothermal bits and components, including grease and elastomer seals, are adequate for temperatures up to 150°C (302°F). Beyond these temperatures, the bit's internal components and lubricating material can degrade causing bearing failure limiting on-bottom drilling hours. In the application, the bottom hole temperature is approximately 180°C (350°F) and in some instances it can exceed 280°C (536°F). The extreme heat reduces on-bottom drilling hours leading to multiple bit runs/trips that drive up development costs. The operator required new roller cone technology that would endure the downhole environment.

To solve this challenge, a series of tests were conducted with temperature resistant elastomers and grease compounds in a controlled laboratory environment. The experiments resulted in a new line of roller cone bits equipped with an innovative bearing system that includes new proprietary composite elastomer seals with Kevlar® fabric and a proprietary high temperature grease formula. These innovations increased seal life, lubricity and load capacity at elevated temperatures for HT/HP applications.

The new geothermal bit technology has been run in the Italian application with outstanding results. Compared to standard roller cone products, the high-temperature bits have greatly increased on-bottom drilling hours while reducing total bit consumption and costly tripping for bit change out. Since successful development of the geothermal project is tied to reducing drilling costs, the new bit technology has significantly improved project economics. The authors will discuss development of the high temperature seal and grease compounds for drilling the granite basement source rock. They will also outline changes to the TCI cutting structure, field application, dull grades and bit performance data.

Background The Larderello area of central Italy (Figure 1) is geologically active and known for its geothermal productivity.1 The first evidence of organized use of the geothermal resource dates back to the 3rd century BC when the Romans used its hot sulfur springs for bathing. In 1817 a group of entrepreneurs led by Francois de Larderel used steam heated cauldrons to extract boric acid (H3BO3) from volcanic mud. The Grand Duke of Tuscany (Leopold II) was a supporter of Larderel's technique and in 1827 built a town for the factory workers named Larderello in honor of Larderel's contribution to the area.2

In 1904 an experiment using steam emerging from surface vents was used to run a rudimentary generator that produced enough electricity to power five light bulbs. It was the first ever practical demonstration of geothermal power. In 1913 the region's first geothermal power plant went into operation and by 1944 five geothermal generating stations were up and running with a combined capacity of 127 MWe.